1. Technical Field
The present invention generally relates to an apparatus and method for assessing retinal damage to an eye and in particular to an apparatus and method for assessing glaucomatous retinal damage to an eye.
2. Related Art
2.1 Glaucoma Epidemiology
Glaucoma is the second most common cause of blindness in the United States, and the single most important cause of blindness among black Americans. About 80,000 Americans are blind from primary open-angle glaucoma (“POAG”). At least 2 million Americans have POAG, but about half are unaware of it. About 5 to 10 million Americans are ocular hypertensives having an elevated intraocular pressure (“IOP”) that places them at increased risk for the development of POAG. The rate at which such ocular hypertensives develop POAG is about 5 to 10 cases per thousand per year, an incidence which increases with age and the level of the IOP.
2.2. Glaucoma Clinical Indicia
Glaucoma refers to a group of diseases of unknown etiology, whose clinical features generally include [i] atrophy of the optic nerve head, [i] visual field defects, and [iii] an elevation in IOP. An elevation in IOP is not a universal feature of glaucoma. Atrophy of the optic nerve head and visual field defects may arise in the setting of an IOP that is within normal limits. This situation obtains in the case of, for example, normotensive glaucoma. Glaucomatous optic nerve head atrophy is generally detected with an opthalmoscope. Glaucomatous visual field defects are generally detected and mapped with a perimeter, and elevations in IOP are generally measured with an applanation tonometer or like device.
2.3 Optic Nerve Head
Using an opthalmoscope, the normal optic nerve head appears as a vertically oriented oval structure in the posterior retina within which a yellow-hued depression, called the optic cup, may be distinguished from a pink-hued neuroretinal rim, and from which retinal vessels course into the retina.
About 1.2 million retinal nerve fibers—axons arising from retinal ganglion cells—are gathered at the optic nerve head to form the optic nerve, which emerges from the back of the eye and extends to the brain. Simplistically, the optic nerve head may be regarded as an anterior axial section through a tube of neural cables that carry the modulated input of the retina used by the brain to create the sense of vision.
2.4 Optic Nerve Head Changes in Glaucoma
Ophthalmoscopic changes in the optic nerve head associated with glaucoma include enlargement, distortion, pallor or deepening of the optic cup, thinning of the neuroretinal rim, the emergence of splinter hemorrhages, and changes in the morphology of retinal blood vessels. These changes are associated with atrophy, scarring and loss of retinal nerve fibers.
2.5 Distinguishing the Visual Field from Visual Acuity
In discussing glaucoma it is important to distinguish visual acuity from the visual field. As an approximation, an eye's visual acuity refers to how sharply it brings things in the world into focus, whereas an eye's visual field refers to how much of the world is seen. Alternatively stated, the visual field refers to the entire area of space that is visualized by an eye while its gaze is fixed.
It possible for an individual to have very keen visual acuity that is limited to a narrow visual field, or a partial visual field outside of which the world does not exist visually. This visual state may be simulated by having a person with normal visual acuity gaze through a narrow pipe or hold a finger over the upper eyelid so that it occludes the superior visual field.
Because glaucoma leads to destruction of retinal nerve fibers carrying the modulated output of retinal photoreceptor cells (as opposed to destruction of the lens or cornea, which form the refractive media of the eye), glaucoma is primarily destructive of a patient's visual fields. Consequently, the detection and mapping of the visual field of each eye is important to the diagnosis and management of glaucoma.
2.6 Visual Field Perimetry
The most widely used technique for assessing glaucomatous damage is subjective visual field testing or perimetry. In perimetry, a patient is seated facing the opening of a hollow hemispheric bowl whose central axis is oriented horizontally. The patient's chin is supported by a chin rest. A constellation of small light sources of variable luminance is arrayed over the inner surface of the hemisphere. One of the patient's eyes is occluded and a trial lens correcting the patient's refractive error is placed in front of the eye to be tested. The patient is instructed to direct the gaze of the open eye onto a fixation target centered at the pole of the hemisphere opposite his or her line of sight, and is further instructed not to allow his or her gaze to drift from the fixation target, relying entirely on peripheral vision to respond to the test. The test is conducted by illuminating the small light sources arrayed across the inner surface of the hemisphere in any of a number of predetermined test strategies. The patient is instructed to press a button on a hand-held device whenever a light flash is discerned anywhere in the field of vision except at the fixation point. The patient's responses are recorded and mapped as a grayscale image depicting visual sensitivities of the retina in shades of gray. Areas of very poor retinal sensitivity are darkly shaded, and areas of good retinal sensitivity are lightly shaded.
Conventional perimetry requires the presence of an operator, and has largely been supplanted by automated perimetry, in which the role of the operator has been assumed by a computer.
2.7 Threshold Strategies
Automated perimeters employ predetermined strategies to test the visual field. Since it is impractical to test every point in the visual field, each strategy uses a grid of test points that covers a circular central area of the visual field that is most likely to show glaucomatous defects. Each point in the grid is tested to determine the visual sensitivity of the central retina to a light stimulus. Just as it is not practical to test each point in the visual field, it is not practical to test any given point in the grid with a wide range of stimulus intensities. Accordingly, threshold strategies have been developed to define a standard level of sensitivity and to find that level with the smallest number of trials.
A stimulus that is bright enough to be easily seen should elicit a response from a patient 100% of time. A stimulus that is too dim to be seen should never elicit a response. Somewhere between there should be a stimulus intensity that will elicit a response from a patient in half of the presentations; and the threshold is defined as that level of light intensity to which a patient responds 100% of the time.
If a stimulus is above (brighter than) the threshold level, it is said to be suprathreshold. If a stimulus is below (dimmer than) the threshold level it is said to be infrathreshold. The threshold stimulus for a point on the retina is determined by exposing it to stimuli above and below a presumptive threshold value in a process called bracketing. The presumptive threshold value is derived from known age-dependent normal values obtained from population studies.
If a first stimulus presentation is suprathreshold, then the computer presents the next stimulus at a level that is, for example, 4 dB lower. If the second stimulus elicits a response, then the computer lowers the stimulus level once again, for example, by 4 dB. This lowering of the stimulus level continues until the stimulus no longer elicits a response, indicating that the threshold has been crossed, and that an infrathreshold stimulus level has been found. The next stimulus is then raised, for example, by 2 dB. If the stimulus elicits no response, then the next stimulus is raised again, for example, by 2 dB. This process continues until the stimulus once again elicits a response in the patient's suprathreshold zone.
Accordingly, the threshold is crossed twice. After the second crossing has occurred, the search is terminated. The threshold value then lies between the visualized suprathreshold stimulus having the lowest intensity and the nonvisualized infrathreshold stimulus having the highest intensity.
2.8 Screening Strategies
Automated perimeter screening strategies are designed to quickly determine whether a significant visual defect is present. If a defect is detected, then a more comprehensive threshold strategy is used to characterize the defect.
Screening strategies use known normal threshold values to present only suprathreshold stimuli that are just above the normal threshold values. If the patient misses a significant number of these stimuli, then the automated perimeter is considered to have detected a defect that warrants further testing.
2.9 Disadvantages of Perimetry
Automated perimetry is beset with a number of disadvantages arising from the subjectivity of the patients' responses and the excessive duration of the test. Patients dislike the test because it is long and tedious. Measurement of the visual field of one eye can take about 10 to 20 minutes. Test anxiety, attention deficits, fatigue and boredom interfere with perimetry in both the young and the elderly. A desire to influence the outcome of the test favorably can result in false positives (pressing the button to signal visualization of a light stimulus when no light has been activated). Attention deficits can result in false negatives (failing to press the button to signal visualization of a light stimulus having a suprathreshold luminance). The results of automated perimetry are often unreliable.
Stimulating a larger retinal area in perimetry with a single large-area light stimulus, instead of using point stimuli, would reduce the duration of perimetry testing. However, the patient's response to a larger-area threshold test in perimetry may be mediated by undamaged retinal areas near damaged retinal areas. In such a case, a patient may see only a portion of the large-area stimulus, but still offer a response that will be falsely indicative of visualization of the entire stimulus. Thus, a large-area threshold test in perimetry is likely to be associated with a substantial risk of a falsely normal test result.
2.10 Pupillary Light Reflex: PLR
The pupillary light reflex (“PLR”) is a clinical sign of the condition of the central nervous system (“CNS”). In a normal patient, exposure of the pupil of one eye to a light stimulus results in a symmetric constriction of both pupils.
2.11 Pupil Perimetry
A quantitative measurement of a PLR may be obtained using an instrument called a pupillometer.
Pupil perimetry usually employs a pupillometer together with a stimulus arrangement similar to that of a perimeter. However, instead of measuring the visual responses to incremental increases over a threshold in the intensity of a light stimulus, pupil perimetry measures the latency and amplitude of the constriction of the pupil in response to a spot (“small-area”) stimulus, with a fixed suprathreshold luminance, that is directed to different locations on the retina.
The pupillary response to spatially-localized luminance increments has been used as an indicator of glaucomatous retinal damage, but the small-area stimuli used in pupil perimetry may target small retinal areas that only weakly stimulate a PLR, and may fail to stimulate a PLR if the small retinal area that is being stimulated by light has been damaged by glaucoma. Large variations in pupil response amplitude among patients and the changes in sensitivity of the pupil response with the retinal location of the small-area light stimulus have also limited the usefulness of such measurements. Additionally, prevailing pupil perimetry takes a relatively long time to perform.
A faster pupil perimetric exam is desirable, especially for purposes of screening large populations for glaucoma.
There is a need for an objective and rapid technology for assessing glaucomatous damage that is not beset with the disadvantages of either pupil perimetry or visual field perimetry.